Previously, `-Dman=false` was the default, because the generated man
pages were shipped in the distribution tarball already, so the option
actually mostly controlled whether to *re*build them.

The generated pages are no longer shipped in the tarball (and probably
haven’t been since the port to Meson, though I haven’t checked), so it
makes sense to change the default to encourage building the man pages if
the right tooling (`rst2man`) is available.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>